Collisional cross-section of water molecules in vapour studied by means of 1H relaxation in NMR.

Daniele Mammoli,Roberto Buratto,Geoffrey Bodenhausen,Pascal Miéville,Estel Canet,Lothar Helm

doi:10.1038/srep38492

Abstract

In gas phase, collisions that affect the rotational angular momentum lead to the return of the magnetization to its equilibrium (relaxation) in Nuclear Magnetic Resonance (NMR). To the best of our knowledge, the longitudinal relaxation rates R1 = 1/T1 of protons in H2O and HDO have never been measured in gas phase. We report R1 in gas phase in a field of 18.8 T, i.e., at a proton Larmor frequency ν0 = 800 MHz, at temperatures between 353 and 373 K and pressures between 9 and 101 kPa. By assuming that spin rotation is the dominant relaxation mechanism, we estimated the effective cross-section σJ for the transfer of angular momentum due to H2O-H2O and HDO-D2O collisions. Our results allow one to test theoretical predictions of the intermolecular potential of water in gas phase.

Highlights

Water is the most extensively studied molecule on Earth
Spin-dependent interactions vary as a function of time and, if the fluctuations occur at frequencies in the vicinity of the nuclear Larmor frequency ω0, longitudinal Nuclear Magnetic Resonance (NMR) relaxation takes place
NMR relaxation studies on the influence of hydrogen/deuterium isotopic substitution on collisional cross-sections have been reported for methane[15,16]

Summary

Introduction

Water is the most extensively studied molecule on Earth. A precise determination of its intermolecular potential would allow accurate predictions of its properties in gas, liquid and solid phase. If spin-rotation is the dominant mechanism, cross-sections for the transfer of angular momentum can be obtained from NMR relaxation rates in gas phase. Such relaxation rates have been measured over a wide range of pressures and temperatures[8,9,10]. ΤJ is the spin-rotation correlation time, Ceff (in Hz) the spin-rotation constant, ωJ the rotational frequency (in rad/s)[46], is the number density of molecules, v is the average thermal velocity, σJ is the collisional cross-section for the transfer of angular momentum, I0 is the moment of inertia, grot is the g-factor, μN is the nuclear magneton, H is the magnetic field and μis the reduced mass of the two colliding particles. DD relaxation can occur between spins in the same molecule (intramolecular DD) or between spins in different molecules (intermolecular DD)

Methods

Results

Conclusion